1. Field of the Invention
The present invention relates generally to a brake apparatus, and is directed more particularly to a brake apparatus for applying braking force to a motor used in a tape recorder, record player or the like.
2. Description of the Prior Art
A prior art brake apparatus used for rapidly stopping a rotating load such as a turntable, reel or the like, which is coupled to the rotary shaft of a motor directly or indirectly, is formed as shown in FIG. 1. That is, an eddy current generating disc 3 such as an Aragos rotating disc is attached to a rotary shaft 2 of a motor 6 to which a load 1 such as a turntable or the like is attached, and a permanent magnet 4 is located near the disc 3 and/or a brake device 5 having a lining 5a is located near the turntable 1. When the rotating load or turntable 1 is desired to be stopped rapidly, the magnet 4 is moved to the disc 3 to brake the rotation of turntable 1 or the lining 5a of brake device 5 is urged to the turntable 1 to brake the latter.
With the above prior brake apparatus, in the case where the load 1 is braked by eddy current generating disc 3, when the rotating speed of load 1 becomes lower, a great braking force can not be generated, while in the case where the mechanical brake device 5 is used, a great braking force is obtained, but sliding or frictional sound is caused, which is undesirable.
Further, if in order to shorten a time period required to change the rotation speed of a rotary body, a brake apparatus is formed to generate a braking force, which can change the rotation speed of a rotating body or stop its rotation rapidly, the inertial moment of rotary body becomes high and hence a time period required to change or stop the rotation of motor is prolonged. As a result, a long time interval is necessary to exchange a record disc of a record player or to operate its pickup arm.
In general, in order to brake a motor by utilizing eddy current as described above and shown in FIG. 2, the motor 6 is disconnected from an AC voltage source 7 and stator or main windings 11 of motor 6 are supplied with DC current from a DC voltage source (not shown), or a series circuit consisting of a diode 13, a resistor 14 and a capacitor 15 is connected in parallel to the main windings 11 of motor 6, auxiliary (split-phase) windings 12 of motor 6 are connected to movable contact 8a of a switch 8 through a capacitor 16, the AC voltage source 7 is connected to fixed contacts 9 of switch 8, the connection point between the capacitor 15 and resistor 14 is connected also to the other fixed contact 10 and both ends of main winding 11 are connected to movable contact 8a and fixed contact 9 of switch 8. Now, if the movable contact 8a is connected to fixed contact 9 of switch 8, the motor 6 is driven. During this period, the capacitor 15 is charged through the diode 13 and resistor 14. While, if the movable contact 8a is connected to the other fixed contact 10, the charge stored in the capacitor 15 is discharged, hence a DC current flows through the main winding 11 of motor 6, and eddy current is produced on the rotor (not shown) of motor 6. Thus, a braking force is applied to motor 6.
FIG. 3 is a graph showing the relation between the rotation speed (r.p.m.) of motor 6 and the braking torque applied thereto when the DC current is fed to the main winding 11 of motor 6, in which the DC voltages 0, 10, 20, 30 and 40 V (volt) are taken as parameters, respectively. From the graph of FIG. 3, it will be apparent that when the rotation speed of motor 6 becomes lower than 200 r.p.m., the braking torque decreases abruptly. Accordingly, it is easily understood that, in an ordinary tape recorder or record player in which its reel or turntable is rotated at a low speed such as 45.about.331/3 r.p.m., the above brake apparatus using DC current has no substantial brake effect. In this case, if DC voltage is increased, a certain amount of braking torque can be produced. However, in this case such a defect is caused that the main winding of motor is damaged.